The present invention relates to a toner for use in a recording method utilizing electrophotography, electrostatic recording, electrostatic printing or toner jetting; and also an image forming method and a process-cartridge using the toner.
In electrophotographic process in general, an electrostatic latent image is formed on a photosensitive member by various means and developed with a toner to form a toner image. The toner image is then transferred onto a transfer(-receiving) material such as paper, as desired, and then fixed, e.g., by heating, pressing or heating and pressing, or with solvent vapor, to obtain a fixed toner image.
Regarding the final step of fixing the toner image onto a sheet (fixation sheet) of, e.g., paper, various methods and apparatus have been developed, and the currently most popular systems adopt a pressure-heating scheme using hot rollers or a fixed heat-generating heater via a heating film.
In the pressure-heating scheme using hot rollers, a fixation sheet carrying a toner image is caused to pass through a heating roller while the heating roller surface and the fixation sheet surface carrying the toner image are caused to contact each other, thereby fixing the toner image onto the fixation sheet. In this method, the heating roller surface and the toner image on the fixation sheet are caused to contact each other under pressure, the heat efficiency for melt-bonding the toner image onto the fixation sheet is very good, thereby allowing quick fixation.
However, as the toner image in a softened and melted state is caused to contact the heating roller surface under pressure, a portion of the toner image can be attached and transferred onto the heating or fixing roller surface and re-transferred to a subsequent fixation sheet to soil the subsequent fixation sheet. This is called an offset phenomenon. The offset phenomenon is largely affected by the fixing speed and the fixing temperature. Generally, in the case of a slow fixing speed, the heating (i.e., fixing) roller surface temperature is set to be relatively low, and in the case of a fast fixing speed, the heating roller surface temperature is set to be relatively high. This setting change is adopted in order to supply a constant amount of heat for fixation to a toner image regardless of the fixing speed.
A toner image on a fixation sheet is composed of a number of toner particle layers. As a result, in the case of a high fixing speed requiring a higher heating roller surface temperature, a large temperature difference occurs between the uppermost toner particle layer directly contacting the heating roller and the lowermost toner particle layer contacting the fixation sheet. A higher heating roller surface temperature is liable to excessively soften and melt the uppermost toner particle layer to result in an offset phenomenon. On the other hand, a lower heating roller surface temperature is liable to fail in sufficiently melting the lowermost toner particle layer for fixation and cause a fixation failure of the toner onto the fixations sheet, thus resulting in a so-called low-temperature offset phenomenon.
For solving the above-mentioned difficulties, it has been generally practiced to increase the fixing pressure in the case of a high fixing speed, thereby anchoring the toner onto the fixation sheet. By this measure, the heating roller temperature can be lowered to some extent, thereby alleviating the high-temperature offset phenomenon. In this case however, a very large shearing force is applied to the toner layer, thereby causing difficulties, such as winding offset of the fixation sheet being wound about the fixing (i.e., heating) roller, and separation claw traces (in the fixed toner image) due to action of separation claws for separating the fixation sheet from the fixing roller. Further, because of a high fixing pressure, e.g., line images are liable to be collapsed or a portion of the toner image is scattered to deteriorate the fixed toner images.
Hitherto, the improvement in toner offset phenomenon and the improvement in toner fixability have been regarded as an identical problem, but the conventional solution therefor by an improvement in molecular weight distribution of toner binder resin and the addition of a low-melting point wax can result in only limited and insufficient levels of improvements in fixability and anti-offset property.
Other trials of improving the releasability of a fixing member and a cleaning member may be effective for achieving a sufficient offset-preventing performance in an initial stage of use but can consequently result in offset phenomenon in a long period of use due to deterioration with time of the members if the releasability of the toner per se is insufficient.
For impart a toner with a releasability, the toner is caused to contain a wax in some cases, but a large amount of wax has to be contained for maintaining a sufficient offset-preventing effect even by using a fixing member and a cleaning member which have been deteriorated with time. In such a case, the toner is liable to suffer from difficulties with its developing performance, such as a lowering in image density and an increase in fog density, and it becomes difficult to control the dispersion state of a wax contained in toner particles, so that the toner is liable to contain a large amount of isolated wax, which is liable to result in toner cleaning failure on the photosensitive member leading to image defects.
More specifically, waxes are added in the toner production stage in order to improve the toner releasability and fixability, but the uniform dispersion of waxes in toner particles is not so easy, and insufficient dispersion of wax is liable to result in problems not only in toner fixability but also in developing performance of the toner. These problems are particularly noticeable in recent development of toners of which the particle size is becoming smaller in recent years.
Regarding proposals in recent years, JP-A 6-118700 has disclosed a toner having tan xcex4 values at room temperature and a high temperature giving a ratio falling within a specific range so as to suppress a lowering in chargeability in a high temperature region, but the dispersibility of a wax in toner particles has not been improved.
JP-A 61-279864 has disclosed a toner having specified shape factors SF-1 and SF-2, and JP-A 63-235953 has disclosed a toner made spherical by application of a mechanical impact force, but the improvements in toner transferability and fixability are insufficient.
JP-A 10-97095 and JP-A 11-202557 have disclosed toners having specific circularity values in order to provide a toner with an improved transferability. JP-A 11-149175 has disclosed a toner surface-treated by application of a mechanical impact force in order to provide improvements in toner transferability, scattering at the time of fixation, etc. These toners have been improved in transferability but have left room for improvement regarding uniform wax dispersion in toner particles.
JP-A 57-171345 has disclosed a developer containing as a binder a copolymer of styrene monomer, (meth)acrylic monomer and unsaturated polyester resin. JP-A 62-195681 has disclosed a developer containing as a principal binder component a polyester resin which contains a specific proportion of vinyl resin having a specific molecular weight and a glass transition temperature. These developers have not been sufficiently improved with respect to fixability and wax dispersibility.
JP-A 11-153885 has disclosed a toner containing a binder resin obtained by reaction between a polyester resin having a specific molecular weight and a vinyl polymer having a specific structure, but the fixability and wax dispersibility have not been sufficiently improved.
JP-A 2000-56511 has disclosed a toner containing a binder resin which contains a hybrid resin component, a specific proportion of insoluble matter within a specific solvent and a tetrahydrofuran-soluble content having a specific molecular weight distribution, but the toner has left a room for improvement regarding the image forming performance in a high temperature/high humidity environment.
JP-A 9-146292 has disclosed a toner containing polyalkylene fine particles having a specific dynamic friction coefficient and providing a fixed toner image showing a specific range of contact angle, and JP-A 9-244294 has disclosed a toner containing specific polyalkylene fine particles having a specific dynamic friction coefficient and having a specific relationship between contact angle and dielectric loss tangent of toner, in order to improve the fixability and fog. The transferability and wax dispersibility of the toners have not been improved, and the improvement in fixability is insufficient.
JP-A 2000-47428, JP-A 2000-47429 and JP-A 2000-47430 have disclosed a toner having specific contact angles in order to improve the toner fixability and reduce the toner attachment onto the fixing member, but the improvement in transferability of the toner is not sufficient.
JP-A 2000-284531 has disclosed a toner having a specific dielectric loss tangent and containing an organic zirconium compound as a charge control agent, but the improvement in transferability of the toner is not sufficient.
An object of the present invention is to provide a toner containing a wax in a good dispersion state within toner particles and excellent in low-temperature fixability and anti-high-temperature offset characteristic.
Another object of the present invention is to provide a toner giving a stable image density in normal temperature/normal humidity and high temperature/high humidity environments, when used in a medium- to high-speed image forming apparatus including a hot roller fixing device or a medium- to low-speed image forming apparatus including a pressure-fixing means comprising a fixed exothermic heater for heating via a heat-resistant film.
Another object of the present invention is to provide a toner comprising toner particles containing a wax in a well-dispersed state and showing improved anti-toner attachment and anti-offset characteristic even with fixing members which have been deteriorated with time.
Further objects of the present invention are to provide an image forming apparatus and a process-cartridge including such a toner as described above.
According to the present invention, there is provided a toner comprising: at least a binder resin, a colorant and a wax, wherein
(a) the toner exhibits a dielectric loss tangent showing a maximum of 6.0xc3x9710xe2x88x922 to 10.0xc3x9710xe2x88x922 in a temperature range of 90 to 125xc2x0 C.,
(b) the toner provides a DSC curve showing at least one heat-absorption peak or shoulder in a temperature range of 85 to 140xc2x0 C. on temperature increase according to differential scanning calorimetry (DSC), and
(c) the binder resin comprises a hybrid resin having a vinyl polymer unit and a polyester unit.
According to the present invention, there is also provided an image forming apparatus, comprising:
(I) a developing step of developing an electrostatic image carried on an image-bearing member with the above-mentioned toner to form a toner image;
(II) a transfer step of transferring the toner image on the image-bearing member onto a recording material via or without via an intermediate transfer member; and
(III) a fixing step of heat-fixing the toner image onto the recording material.
The present invention also provides a process-cartridge detachably mountable to a main assembly of an image forming apparatus for forming a toner image by developing an electrostatic latent image formed on an image-bearing member,
wherein said process-cartridge includes (i) an image-bearing member, (ii) a developing means for developing an electrostatic latent image carried on the image-bearing member with the above-mentioned toner to form a toner image on the image-bearing member, and (iii) at least one means selected from the group consisting of a charging means for charging the image-bearing member, a latent image-forming means for forming the electrostatic latent image on the image-bearing member, a transfer means for transferring the toner image onto a recording material, and a cleaning means for removing a portion of toner remaining on the image-bearing member after transfer of the toner image onto the recording material.